The invention relates to embossing, especially to a device used to preheat a thermoplastic embossable material, prior to contact with an embossing roll.
It is well known in the art to pass such material from a heat drum downwardly between a pair of radiant heaters into a nip formed between an embossing roll and a backup or bed roll. It has been found that air or gaseous vapors form under the sheet of material next to the surface of the heat drum, as the material is heated. The accumulation of such vapor forms under the sheet, bubbles which interfere with the uniform heating of the material by the heat drum and radiant heaters causing lower embossing depths which are referred to as embossing streaks.
Moreover, once a vinyl-type product is heated to the yield point for embossing, any variation in tension in the sheet can cause distortion of the material. This distortion is particularly bad in geometric patterns that have to have straight lines and configurations. Heat drums have to be large enough to provide necessary dwell time for proper heat transfer to the material. However, use of a larger drum results in more problems of inertia and speed control. Minute speed variations can alter the tension to cause distortion of the material.
Then too, the temperature of a heat drum can vary from end-to-end to cause a temperature differential in the material which, in turn, causes a variation in the embossment of the material resulting in an unacceptable appearance and feel of the finished product.
U.S. Pat. No. 3,950,480 relates to a method of embossing plastic material without the use of a heat drum. Instead, a pair of hot air heaters 14, 15 are used to direct streams of hot air against opposing sides of the material, as the sole method of heating the material for embossing. For some materials, it can be imagined that the heating zone between the heaters would have to be quite lengthy to properly heat the material. Also, the streams of hot air would have to be carefully controlled, so as not to adversely distort the material as it is struck by the streams of air.
U.S. Pat. No. 4,556,378 relates to a pair of adjacently disposed rotatable discs 2, 3, the first disc 2 to be encountered by the material, carrying a circumferential band which is used to preheat the material, prior to contacting the second, molding disc 3. It can be imagined that the rotational speeds of the two discs must be carefully controlled to prevent distortion of the material.
U.S. Pat. No. 2,776,451 relates to an embossing system which utilizes a heated, perforated metal conveyor for supporting and heating the material as it is transported to the nip between the embossing roll 43 and backup roll 42. Again, the speed of the conveyor must be carefully synchronized with the rotation of the embossing roll to prevent distortion of the material.
The invention is directed to the provision of an embossing apparatus in which many of the aforementioned problems are eliminated or substantially reduced.
Briefly stated, the invention is in an apparatus and related process for embossing a heated thermoplastic material, such as a scrim or paper backed vinyl. The apparatus comprises a perforated, fixed metal platen which is curved downwardly towards the nip formed between the embossing roll and backup roll, such that the material will be slidably supported on the platen. Means are provided for simultaneously heating the platen and the outer surface of the material farther spaced from the platen than the underside or backing of the material contacting the platen. The scrim or paper backing of the vinyl must not be exposed to the high temperatures to which the vinyl is exposed, because of the instability of such backing or substrate.